Electronic filters are circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both. There are different types of filters including analog or digital, active or passive, high-pass, low-pass, band-pass, band-reject, or all-pass, discrete-time (sampled) or continuous-time linear or non-linear infinite impulse response (IIR type) or finite impulse response (FIR type) and the like.
Passive filters make use of only passive components like resistors, capacitors and inductors whereas in Active filters, along with the aforesaid components, active components like Operational Amplifiers (opamps), Field Effect Transistors (FETs), Operational Tranconductance Amplifiers (OTAs) and the like are used. Active filters are implemented using a combination of passive and active (amplifying) components, and require an outside power source. Although active elements of the filters provide high linearity and a quality factor along with sharp resonance, their bandwidth is limited.
Integrated continuous-time filters have largely been realized using Transconductance-capacitance (Gm−C), active resistance capacitance (active-RC) or Transconductance−Operational Transconductance Amplifier-Capacitance (Gm−OTA−C) techniques. FIG. 1 illustrates a Gm−C integrator topology known in the art. Gm−C filters are capable of high speed operation due to the open loop nature of the integrators. However, good linearity is usually concomitant with high excess noise, resulting in poor power efficiency to achieve a given dynamic range. As such, their linearity is limited, and they add more noise than necessary from fundamental considerations. They are also sensitive to stray capacitances, as these parasitic capacitances appear in parallel with the integrating capacitors. These aspects have relegated such filters to those applications operating at high speeds but needing a low dynamic range.
FIG. 2 illustrates an active-RC integrator topology known in the art. Active-RC filters are attractive due to their low noise, high linearity and insensitivity to parasitic components, provided an opamp with a sufficiently high gain and bandwidth can be realized. Since every filter node is either a virtual ground or the output of an opamp, active-RC filters are largely insensitive to stray capacitances. In low voltage CMOS processes, it is difficult to realize an opamp with low output impedance that is capable of high swing operation. Therefore, OTA (Operational Transconductance Amplifier) with a sufficiently large transconductance are used in lieu of the opamp. Realizing a high speed OTA remains a key challenge. If the integrating resistor and capacitor as illustrated in FIG. 2 are linear, the only mechanism that results in filter distortion is the nonlinearity of the OTA. Due to the limitations on the realizable transconductance/bandwidth of the OTA, active-RC filters have largely been restricted to applications in low/moderate frequency range.
FIG. 3 illustrates a Gm−OTA−C integrator topology known in the art. The Gm−OTA−C technique is a hybrid of the Gm−C topology illustrated in FIG. 1 and the active-RC architecture illustrated in FIG. 2. The Gm OTA−C technique reduces stray sensitivity and DC-gain problems associated with a Gm−C integrator. It however inherits the dynamic range issues including poor linearity and noise problems of the Gm−C design. Speed wise, Gm−OTA−C integrators are poorer than their Gm−C prototypes (due to the extra delay associated with the OTA), but perform better than their active-RC counterparts. Such an integrator always consumes more power (due to the OTA) than the Gm−C integrator it is based on, while having the same dynamic range. This results in a poor efficiency which is lower than that of the Gm−C integrator.
Therefore there is felt a need for a low distortion active filter with:                combined advantages of active-RC and Gm−C integrators; and        higher speed and linearity operation as compared to the conventional active-RC design.        